1. Field of Invention
This invention deals with digital speech transmission and more particularly with means for efficiently processing speech signals to enable effective use of channel bandwidth.
2. Technical Background
In view of the high cost of transmission channels, it might be wise to take advantage of any speech characteristics which would enable concentrating the traffic of a maximum number of telephone users over a same channel.
During a telphone conversation, each subscriber speaks less than half the time during which the connection is established. The remaining idle time is devoted to listening, gaps between words and syllables, and pauses. A number of systems have already been proposed which take advantage of this idle time. For instance, additional users, up to twice the overall channel capacity, are assigned to a channel in TASI systems (see J. Campanella "Digital TASI", Comsat Technical Revue of 1975). These systems obviously need means for detecting user inactivity to be able to assign the channel to a different user. Unfortunately, the voice activity determination is far from being straightforward. In general, the method for detecting voice activity of a given speaker is based on measurement of the speech s1gnal energy over short periods of time. The measured energy is then compared with a pre-specified threshold level. Speech is declared present if the measured energy exceeds the threshold, otherwise the period is declared idle, i.e. the concerned speaker is considered silent for said period of time. The problem lies with the threshold determination due to the fact that different speakers usually speak at different levels and also due to the fact that the losses vary from one transmission line to another. A threshold set too high would result in the clipping of speech signals. Thus, the received speech signal would be of rather poor quality. While a threshold set too low would obviously lead to degraded system efficiency, in addition, one has also to take into account the presence of noise which should be discriminated from voice signals.
An object of the present invention is to provide for an improved speech activity detection process.
Another object of the invention is to provide for means for detecting low level speech activity in the presence of high level background noise.